stm32f303: Add lacking SoC-level spi dt-node description.
stm32f303: Add lacking SoC-level spi dt-node description.

The stm32f303 SoC has a spi3 peripheral not listed in current dts.
Add lacking description.

Signed-off-by: Jan Kuliga <ce5003e3901258e8edf4fbec3f5202410d52911a@gmail.com>

galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr